Chain Length Effects in Boundary Lubrication

The effect of additive chain length on friction coefficient and transition temperature is investigated under different test conditions. Carboxylic acids and normal alcohols with 12 – 18 carbon atoms in the chain were used as additives in n-hexadecane base lubricant. It was found that the magnitude of the friction coefficient decreases as the additive chain length is increased. Matching of the additive chain length with the base lubricant had no effect on the friction coefficient. Similarly, the transition temperature at low sliding speeds increased with increasing chain length and was not affected by chain matching. At higher speeds, the acids containing 16 and 18 carbon atoms did not exhibit a transitional behavior within the studied temperature range. These results are discussed in terms of the separation distance between the contacting surfaces, the intermolecular dispersion forces and the structure of adsorbed layers. It is concluded that, at very low speeds, boundary lubrication is controlled by adsorbed monolayers, whereas at higher speeds, where partial elastohydrodynamic conditions prevail, ordered multimolecular layers may influence lubrication.